JP2002155317A - Method for manufacturing high strength galvanized steel sheet having excellent deep drawability and secondary working brittleness resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength galvanized steel sheet which has excellent deep drawability, secondary working brittleness resistance and plating characteristics, and further has a tensile strength of about >=400 MPa. SOLUTION: A steel slab having a composition containing, by mass, 0.0005 to 0.008% C, 0.1 to 1.5% Si, 0.5 to 3.0% Mn, 0.02 to 0.2% P, <=0.02% S, 0.005 to 0.20% Al, <=0.01% N, 0.0005 to 0.008% B, 0.05 to 2.0% Mo and Nb of 0.001 to 0.2%, and also, in the range satisfying 0.3×(C/12)<=Nb/93<=3.0×(C/12) is heated and soaked at a temp. of 950 to 1,300 deg.C. After hot rolling is finished at a temp. of 650 to 1,000 deg.C, the steel is coiled at a temp. of 400 to 850 deg.C. The steel is next annealed in the temperature range of 600 to 900 deg.C for >=10 min as black film scale is adhered thereto. The steel is pickled and is subjected to cold rolling at a draft of 50 to 95%. Thereafter, the steel sheet is subjected to recrystallization annealing at a temp. of 700 to 950 deg.C and is subjected to galvanizing in continuous galvanizing equipment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet suitable for use in applications where bending, press forming, drawing and the like are performed, such as a steel sheet for an automobile body. It is about.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for reducing the weight of a vehicle body in order to improve fuel efficiency due to regulations on exhaust gas from automobiles due to environmental problems. Also, improving the safety of automobiles has become an important issue. Therefore, as one of the measures against such a problem, a high-strength hot-dip galvanized steel sheet having a tensile strength of about 400 MPa or more and excellent press formability is required.

[0003] However, cold-rolled steel sheets generally have a deep drawability, that is, a Rankford value (r value) in accordance with the increase in strength.
In addition, the balance between the tensile strength (TS) and the ductility (El) tends to deteriorate, and the surface properties such as plating properties also tend to deteriorate. Therefore, in order to provide a steel sheet for automobiles, it is important to improve the deep drawability and the plating properties as well as to increase the strength.

[0004] Various methods have been proposed to improve the deep drawability, which has tended to deteriorate with increasing strength. For example, in JP-A-63-100158, based on an ultra-low carbon steel with reduced C, in order to improve workability and aging, Ti, Nb and the like, which are carbonitride forming components, are added. S, an element that does not adversely affect processability
A high-strength cold-rolled steel sheet has been proposed in which the strength is mainly increased by adding i, Mn, and P, thereby improving the formability.

Although Si is an effective component for increasing the strength without deteriorating the r value or El, if a large amount of Si is contained, deterioration of the surface characteristics is unavoidable, and the plating characteristics are significantly deteriorated. There is a problem of doing. P is
In particular, ultra-low carbon steel has a problem of deteriorating secondary work brittleness. Furthermore, Mn has a problem that, depending on the content of Mn, inconveniences such as deterioration of plating characteristics and deterioration of material occur.

Further, Japanese Patent Application Laid-Open No. Hei 5-339641 discloses that Nb is added to an ultra-low carbon steel and Si,
A method for producing a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet in which r-value is improved by lubricating hot-rolling a steel to which an appropriate amount of Mn and P is added in a ferrite region. According to this technique, although it is possible to produce a high-strength steel sheet for deep drawing having a tensile strength of about 400 MPa or more and a high r-value, lubricating rolling must be performed during hot rolling, There is a possibility that problems such as slippage during rolling and poor biting may occur. Further, there is no mention of the above-mentioned deterioration of plating characteristics due to the addition of Si, and there is no description about plating characteristics.

On the other hand, as means for increasing the strength of a steel sheet for galvannealing, as disclosed in JP-A-5-255807, Si is limited to 0.03 wt% or less, and A method mainly using P and Mn was generally used.
However, there is a problem that addition of a large amount of P delays alloying of a hot-dip galvanized steel sheet and deteriorates secondary work brittleness resistance. Mn also has a small effect on plating characteristics, but when Si is limited to 0.1 wt% or less, plating characteristics begin to deteriorate when the amount of Mn is 1 wt% or more, and transformation is caused when a large amount is contained. There is a problem that the points are lowered and the hot-rolled sheet is hardened, and inconveniences such as not recrystallizing at the time of annealing, which lead to deterioration of the material occur.

[0008] With respect to the deterioration of the secondary work brittleness resistance, a method of adding B is generally known as a means for improving the brittleness. Further, Japanese Patent Application Laid-Open No. 10-17994 discloses that the addition of S and N is aggressive. The resistance of alloyed hot-dip galvanized steel sheet is 2
A technique for improving the secondary working brittleness is disclosed. However, both the S and N components tend to form compounds during the slab heating to hot rolling to winding steps. For example, the S component includes compounds such as TiS and MnS,
Also, the N component tends to form compounds such as TiN, AlN and BN, respectively.

As described above, there is a natural limit in achieving high strength while maintaining the deep drawability, the resistance to secondary working embrittlement, and the plating characteristics only by adjusting the amounts of P, Mn, and B in steel.

[0010]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, has a tensile strength of about 400 MPa or more, has excellent deep drawability and resistance to secondary working brittleness,
It is another object of the present invention to propose an advantageous method for producing a high-strength hot-dip galvanized steel sheet having excellent plating characteristics.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have utilized Si, Mn, and P as reinforcing components and added Nb as a carbide-forming element. In order to achieve a balance between deep drawability and plating characteristics in such a steel type, a black scale was attached to the hot-rolled sheet to make it compatible with Mo. It has been found that it is extremely effective to perform the above steps. The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows. 1. C: 0.0005 to 0.008 mass%, Si: 0.1 to 1.5 mass
%, Mn: 0.5 to 3.0 mass%, P: 0.02 to 0.2 mass%,
S: 0.02 mass% or less, Al: 0.005 to 0.20 mass%, N: 0.
01 mass% or less, B: 0.0005 to 0.008 mass%, Mo: 0.05 to
2.0 mass%, Nb: 0.001 to 0.2 mass%, and 0.1 mass%.
A steel slab containing 3 × (C / 12) ≦ Nb / 93 ≦ 3.0 × (C / 12) with the balance being substantially the composition of Fe and unavoidable impurities at 950-1300 ° C. After heating and soaking, finish hot rolling at 650-1000 ° C, then 400-85
Wound at 0 ° C, then 600 scale with black scale attached
Anneal for more than 10 minutes in a temperature range of ~ 900 ° C,
Deep drawability characterized by pickling, cold rolling at a reduction rate of 50 to 95%, then recrystallization annealing at 700 to 950 ° C in a continuous hot dip galvanizing facility and then hot dip galvanizing. And a method for producing a high-strength hot-dip galvanized steel sheet having excellent secondary work brittleness resistance.

2. In the above item 1, the steel slab further comprises
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized by having a composition containing Sb: 0.001 to 0.03 mass%.

3. In the above 1 or 2, the steel slab further contains Ti: 0.002 to 0.05 mass%, and Ti / 48 ≦ 1.5 ×
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized in that the composition contains a composition satisfying (N / 14 + S / 32).

4. In the above 1, 2 or 3, the steel slab further contains 0.02 to 2.0 mass% of Cu and 0.02 to 2.
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized in that the composition contains one or two selected from 0 mass%.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the results of research on which the present invention is based will be described. 3 having the component composition shown in Table 1
The sheet bars of the steels A to C were heated and soaked at 1250 ° C., and then subjected to three-pass rolling so that the finishing temperature was 900 ° C. to obtain a hot-rolled sheet having a thickness of 3.5 mm. Next, the hot-rolled sheet with the black scale attached thereto was annealed for 1 hour in a temperature range of 500 to 1000 ° C., and then pickled. Then, after cold rolling at a rolling reduction of 80%, recrystallization annealing at 850 ° C and 40s was performed, and then quenched to a temperature range of 450 to 500 ° C.
Was immersed in a hot-dip galvanizing bath containing 0.13 mass% to perform plating, and then an alloying treatment was performed at a temperature of 450 to 550 ° C (Fe content in the plating layer: about 10 mass%).

[0017]

[Table 1]

FIG. 1 shows the results obtained by examining the effects of the steel composition, particularly the Nb component and the hot-rolled sheet annealing temperature, on the r value. Steel A is Nb-added steel and steel B is Nb-free steel. It is.
The r value is an average value in three directions of r _L (rolling direction), r _D (45 ° with respect to the rolling direction), and r _C (90 ° with respect to the rolling direction). R value = (r _L + 2r _D + r _C) ) / 4.

From FIG. 1, it can be seen that a high r-value is obtained by setting the hot-rolled sheet annealing temperature to 600 to 900 ° C. in Nb-added steel A. On the other hand, in the case of steel B not containing Nb, a high r value could not be obtained even when the hot-rolled sheet annealing temperature was changed.

Next, the effect of the steel composition, particularly the Mo component, on the resistance to secondary working embrittlement of the hot-rolled sheet after annealing was examined. FIG.
Measured the brittle transition temperature (Tcr), which is known as a factor indicating the quality of secondary working brittleness resistance, using steel A which is a Mo-added steel and steel C which is a Mo-free steel. Is plotted against the hot rolled sheet annealing temperature.

The resistance to secondary working brittleness of the hot-rolled sheet was determined by punching a 78 mmφ test piece from the annealed hot-rolled sheet, and then drawing out the test piece with a 38 mmφ ball-head punch to obtain a cup. Was cut at a depth of 35 mm, kept at various temperatures (temperature error: ± 1 ° C) for 10 minutes or more, placed on a test table sideways, and the weight of the cup (weight or weight) A drop weight test with a weight of 5 kg and a drop height of 80 cm was performed, and the lowest temperature at which cracks did not occur was measured as Tcr, and evaluation was made from this measured value. The pass / fail judgment of the resistance to secondary working embrittlement was made when Tcr was -45 ° C or less, and was rejected when Tcr exceeded -45 ° C.

FIG. 2 shows that the steel A, which is the Mo-added steel, is 600-600%.
It can be seen that the secondary work embrittlement resistance is excellent over the hot rolled sheet annealing temperature range of 900 ° C. On the other hand, steel C, which is Mo-free steel
Except for the hot rolled sheet annealing temperature of 600 ° C, the secondary embrittlement resistance was not at an acceptable level.

Here, the effects of the steel composition and the hot-rolled sheet annealing on the r value and the secondary workability are considered as follows.

That is, with respect to the r value, when P is contained as in the steel of the present invention, a compound of Ti and P is formed in a Ti-added steel at the time of hot-rolled sheet annealing at 600 ° C. or more, and then cold-rolled. -The r value decreases because the formation of {111} recrystallized texture is inhibited during the annealing process. In this regard, in Nb-added steel, P compounds are less likely to be formed than in Ti-added steel,
In addition, since NbC is coarsened by high-temperature annealing at 600 ° C. or more, {111} recrystallization texture is strongly developed after cold rolling and annealing, and the r value is improved. If the annealing temperature exceeds 900 ° C., the r-value drops sharply because the hot-rolled sheet crystal grains grow abnormally.

With respect to the resistance to secondary working embrittlement, in the case of a steel having an extremely low carbon content, such as the steel of the present invention, to which P, B, and Nb are added, BN is required in the steps of slab heating, hot rolling, and winding. Is generated, and in the subsequent hot-rolled sheet annealing step, the grain boundary segregation of P is promoted. As a result, in the steel sheet after the hot-rolled sheet annealing, due to the grain boundary segregation of P, which is the cause of grain boundary embrittlement, and the decrease in the amount of solute B, the steel sheet has a resistance of 2%.
The subsequent work brittleness is remarkably reduced, which causes the steel sheet to break during cold rolling, which may hinder production. Further, BN remains even during recrystallization annealing after cold rolling and after plating alloying, so that the secondary work embrittlement resistance also decreases in alloyed hot-dip galvanized steel sheets.

In this regard, it has been clarified that Mo-added steel has an effect of improving the resistance to secondary working brittleness. The reason is not clear at present, but since it does not affect the amount of P and B grain boundary segregation, Mo has a function of reducing the action of P segregated at the grain boundary to embrittle the grain boundary. It is considered something.

Although the plating characteristics are not particularly shown in the figure, even when the Si content is as high as 0.7 mass%, it is necessary to perform the hot-rolled sheet annealing with the black scale remaining attached. It was confirmed that the higher the hot-rolled sheet annealing temperature, the lower the non-plating ratio of the steel of the present invention produced, and that high-temperature annealing at 600 ° C. or higher provided plating properties that had no practical problems. The reason that the plating property is improved by performing the hot-rolled sheet annealing while the black scale is still adhered is not necessarily clarified, but the hot-rolled sheet annealing with the black scale remains as it is. It is considered that an oxide is formed on the surface layer of the hot-rolled sheet, and this oxide suppresses the surface concentration of Si.

Next, the reason why the composition range of the steel material is limited to the above range in the present invention will be described. C: 0.0005 to 0.008 mass% The smaller the C content, the better the deep drawability is improved. However, if the C content is 0.008 mass% or less, there is no significant adverse effect.
On the other hand, even if it is less than 0.0005 mass%, no further improvement in deep drawability is observed, and only an increase in steelmaking cost is caused. Therefore, the C content is limited to the range of 0.0005 to 0.008 mass%.

Si: 0.1-1.5 mass% Si is an element that has the effect of increasing the strength without significantly deteriorating the deep drawability, and is added in a necessary amount according to the desired strength. However, when the Si content is less than 0.1 mass%, the effect of the addition is poor. On the other hand, when the Si content exceeds 1.5 mass%, not only the deep drawability is deteriorated, but also the plating characteristics are deteriorated. %.

Mn: 0.5 to 3.0 mass% Mn is an element that has the effect of strengthening steel and is added in a necessary amount depending on the desired strength. However, if the Mn content is less than 0.5 mass%, the effect of improving strength is reduced. On the other hand, if the content exceeds 3.0 mass%, the deep drawability is deteriorated.
%.

P: 0.02 to 0.2 mass% P is an element that has the effect of increasing the strength without significantly deteriorating the deep drawability, and is added in a necessary amount according to the desired strength. However, if the P content is less than 0.02 mass%, the effect of the addition is poor. On the other hand, if the P content exceeds 0.2 mass%, deep drawability is deteriorated. Therefore, the P content is limited to the range of 0.02 to 0.2 mass%.

S: 0.02 mass% or less The smaller the amount of S, the better the deep drawability is. Therefore, it is desirable to reduce the amount as much as possible. However, if the content of S is 0.02 mass% or less, the amount of S is not so adversely affected. % Or less.

Al: 0.005 to 0.20 mass% Al is a useful element for improving the yield of carbonitride forming elements by deoxidation. However, if the Al content is less than 0.005 mass%, the effect of its addition is poor. On the other hand, even if it is added in excess of 0.20 mass%, a further deoxidizing effect cannot be obtained.
The amount was limited to the range of 0.005 to 0.20 mass%.

N: 0.01 mass% or less It is desirable to reduce the N content as much as possible because the smaller the N content, the better the deep drawability is. Therefore, if the N content is 0.01 mass% or less, the N content is not so adversely affected. mass% or less.

B: 0.0005 to 0.008 mass% B is an element that has the effect of improving the secondary work brittleness resistance by segregating at the grain boundaries. However, when the B content is less than 0.0005 mass%, the effect of the addition is poor. On the other hand, when the B content exceeds 0.008 mass%, the effect reaches saturation and rather leads to deterioration of deep drawability.
Limited to the range of 008 mass%.

Mo: 0.05 to 2.0 mass% Mo is an important element in the present invention, and has an effect of increasing the strength without deteriorating the plating property and has an effect of improving the resistance to secondary working brittleness. However, if the Mo content is less than 0.05 mass%, the effect of the addition is poor, while 2.
When the content exceeds 0 mass%, the effect reaches saturation, and rather, the deep drawability is deteriorated. Therefore, the Mo amount is limited to the range of 0.05 to 2.0 mass%.

Nb: 0.001 to 0.2 mass% and 0.3 × (C /
12) ≦ Nb / 93 ≦ 3.0 × (C / 12) Nb is an important element in the present invention.
Is reduced by precipitation and fixation as NbC.
There is an effect that the 11 ° recrystallization texture is developed to improve the deep drawability. However, the Nb content is 0.001mass%
If less, the effect of the addition is poor, while 0.2 mass%
On the contrary, when it exceeds, the deep drawability is deteriorated. Also, the Nb amount
If it is less than 0.3 × (C / 12), a large amount of solute C remains in the steel, so that the {111} recrystallized texture does not develop during recrystallization annealing, and the r value deteriorates. On the other hand, the Nb amount is 3.0 × (C /
If it is larger than 12), a large amount of solid solution Nb remains, and Nb forms a compound with P at the time of hot-rolled sheet annealing to deteriorate the r value. Therefore, the amount of Nb is 0.001 to 0.2 mass% and 0.3 × (C / 1
2) Limited to a range satisfying ≦ Nb / 93 ≦ 3.0 × (C / 12).

Although the essential components have been described above, the present invention may optionally contain the following elements as required.

Sb: 0.001 to 0.03 mass% Sb is an element that, by concentrating on the surface, not only effectively prevents nitriding during hot-rolled sheet annealing but also effectively contributes to improvement in plating property. It is. However, the Sb content is 0.
If less than 001 mass%, the effect of the addition is poor.
Even if it is added in excess of mass%, the effect reaches saturation and conversely leads to deterioration of deep drawability, so Sb is 0.001 to 0.03 mass%.
%.

Ti: 0.002 to 0.05 mass% and Ti / 48 ≦ 1.5
× (N / 14 + S / 32) Ti is a useful element that precipitates and reduces solid solution N and S in steel as TiN and TiS, and improves deep drawability. However, when the Ti content is less than 0.002 mass%, the effect of the addition is poor. On the other hand, when the Ti content exceeds 0.05 mass% or Ti / 48> 1.
At 5 × (N / 14 + S / 32), a compound of Ti and P is formed during annealing of a hot-rolled sheet.
The development of 1｝ recrystallized texture is suppressed, leading to a decrease in the r value. Therefore, the Ti content is 0.002 to 0.05 mass% and Ti / 48 ≦
The range was limited to satisfy 1.5 × (N / 14 + S / 32).

Cu: 0.02 to 2.0 mass%, Ni: 0.02 to 2.0
Both mass% Cu and Ni are elements that have the effect of increasing the strength without deteriorating the plating properties. However, if the content of Cu and Ni is less than 0.02 mass%, the effect of the addition is ineffective, and if the content exceeds 2.0 mass%, the deep drawability deteriorates. Therefore, both are limited to the range of 0.02 to 2.0 mass%. .

Next, each manufacturing process of the present invention will be described. Hot Rolling Step In the present invention, after the steel slab having the above composition is heated and soaked at 950 to 1300 ° C, it is necessary to finish hot rolling at 650 to 1000 ° C and wind it up at 400 to 850 ° C. When heating and soaking a steel slab, a lower treatment temperature is advantageous in depositing and fixing solid solution C and N as carbonitrides. When the heating and soaking temperature of the slab exceeds 1300 ° C., It becomes difficult to precipitate and fix the molten C and N as carbonitrides, and the carbonitrides precipitated during hot rolling become very fine. This is because the coarsening cannot be sufficiently performed even in the annealing step, the cold rolling, and the annealing step, and as a result, during the recrystallization annealing, the development of the {111} recrystallized texture is suppressed, and the r value decreases. . Therefore, the heating-soaking temperature (SRT) of the slab is 13
The temperature was limited to 00 ° C or lower. In order to further improve the workability, the temperature is desirably 1250 ° C. or lower. Further, even if the heating-soaking temperature is lower than 950 ° C., no further improvement in workability is observed, and rather, there is a concern that a rolling trouble may occur due to an increase in the rolling load during hot rolling. Therefore, the lower limit of the heating-soaking temperature was set to 950 ° C.

The finishing temperature during hot rolling (FDT)
Is, Ar ₃ transformation point or more γ region may be any (austenite region) or Ar ₃ transformation point of α region (ferrite region), when the finishing temperature is too high, the crystal grains of the hot rolled sheet becomes coarse, deep drawing The property is deteriorated. On the other hand, if the finishing temperature is too low, the rolling load at the time of hot rolling increases, so the finishing temperature was limited to the range of 650 to 1000 ° C. For the purpose of refining the crystal grains of the hot-rolled sheet, the finishing temperature is 80
It is preferably between 0 and 1000 ° C. More preferably, it is in the range from the Ar ₃ transformation point to 1000 ° C. In addition, in order to refine the crystal grains of the hot-rolled sheet by hot rolling, the total draft during hot rolling is preferably 70% or more.

Further, the coil winding temperature (CT) after hot rolling is not only advantageous for the above-mentioned carbonitride coarsening as the temperature increases, but also because the black scale is thickened, When the strip annealing is performed, a large amount of oxide is formed on the surface layer portion of the hot-rolled sheet, and the surface concentration of Si can be prevented, which is advantageous for improving the plating property. Here, when the winding temperature is less than 400 ° C., the effect is not obtained. On the other hand, when the winding temperature is more than 850 ° C., the crystal grains become too coarse, and conversely, the r value decreases.
The coil winding temperature was limited to the range of 400-850 ° C.
The coil winding temperature is more preferably 600 to 850 ° C.
It is. Further, the slab of the steel of the present invention may be obtained by cooling a continuously cast product once cooled to the Ar ₃ transformation point or lower, or may be heated or maintained without cooling to the Ar ₃ transformation point. Needless to say, a heated one may be used.

Hot Rolled Sheet Annealing Step This step is extremely important in obtaining a high r value and improving the plating characteristics of the Si-containing steel, and the temperature range of 600 to 900 ° C. with the black scale attached. Need to be annealed for more than 10 minutes. That is, even if annealed after hot-rolled sheet pickling, since the oxide that suppresses the concentration of Si is not formed on the surface layer portion of the hot-rolled sheet, the plating characteristics are not improved. By performing the as-annealing, an oxide is formed on the surface layer of the hot-rolled sheet, and the oxide suppresses the surface concentration of Si, resulting in improved plating characteristics. If the annealing temperature is less than 600 ° C,
Since the NbC does not increase in coarseness, the r value does not improve. On the other hand, when annealing is performed in a temperature range exceeding 900 ° C., abnormal grain growth occurs in the hot-rolled sheet, so that the r-value deteriorates. Therefore, the conditions of hot-rolled sheet annealing were limited to the range of 600 to 900 ° C. Also, if the annealing time is less than 10 minutes, the coarsening of NbC does not proceed sufficiently,
In the present invention, the time is limited to 10 minutes or more. The annealing atmosphere may be arbitrary, and may be, for example, a normal nitrogen atmosphere or a hydrogen atmosphere.

Cold Rolling Step This step is necessary to obtain a high r value, and for that purpose, the cold rolling reduction must be 50% or more. That is, if the rolling reduction is less than 50%, excellent deep drawability cannot be obtained. However, if the rolling reduction is too large, the r-value will be reduced, so the upper limit of the rolling reduction is 95%.
And

Annealing / Hot Dip Galvanizing Step The cold rolled steel sheet that has undergone the cold rolling step needs to be subjected to recrystallization annealing. This recrystallization annealing is usually performed in a continuous hot-dip galvanizing line, and the annealing temperature needs to be 700 to 950 ° C. In other words, if the annealing temperature is lower than 700 ° C, recrystallization is not completed, so that excellent deep drawability cannot be obtained.
If it is higher than this, annealing in the γ region will occur and deep drawability will deteriorate. Then, after annealing, it is preferable to rapidly cool to a temperature range of 380 to 530 ° C. If the quenching stop temperature is less than 380 ° C, non-plating may occur, while if it exceeds 530 ° C, the plating surface tends to be uneven.

Subsequent to the heat treatment, plating is performed by dipping in a hot-dip galvanizing bath. At this time, it is preferable that the Al concentration in the plating bath is about 0.12 to 0.145 mass%. In the bath
If the Al content is less than 0.12 mass%, alloying tends to proceed too much and the plating adhesion (powdering resistance) tends to deteriorate,
On the other hand, if it exceeds 0.145 mass%, non-plating tends to occur. When the alloying by heating is performed subsequent to the plating, the Fe content in the plating layer is 9 to 12 mass%.
%.

The steel sheet (specifically, steel strip) after galvanizing may be subjected to temper rolling of 10% or less for shape correction and adjustment of surface roughness and the like. In addition, in the method of the present invention, a special treatment is performed after galvanizing to form a chemical conversion treatment, weldability,
Further improvement in press formability and corrosion resistance can be achieved. The above description is only an example of the embodiment of the present invention, and various changes can be made within the scope of the claims.

[0050]

EXAMPLE A steel slab having the composition shown in Table 2 was prepared as shown in Table 3.
After a hot-rolled steel strip having a thickness of 3.5 mm was formed under the hot-rolling conditions shown in (1), batch annealing was performed while the black scale was still attached or after pickling. Batch annealing was performed only in No. 3 in Table 3 in a hydrogen atmosphere, and the rest was performed in a nitrogen atmosphere. Then, after pickling, a cold-rolled steel strip having a sheet thickness of 0.7 mm was formed by cold rolling, and then subjected to recrystallization annealing and galvannealing in a continuous hot-dip galvanizing line.
The plating bath temperature is 460 to 480 ° C, the plate temperature during immersion is not less than the plating bath temperature and not more than (plating bath temperature + 10 ° C), and the alloying treatment is heated in the temperature range of 480 to 540 ° C for 15 to 28 seconds. Performed by holding. Thereafter, the steel strip was subjected to a 0.7% temper rolling.

The material properties of the hot-dip galvanized steel sheet thus obtained are yield stress (YS), tensile strength (TS),
Table 3 also shows the results of investigations on ductility (EI), Rankford value (r value), Tcr of hot-rolled sheet and hot-dip coated sheet, and plating characteristics. The tensile properties are based on JIS
It measured using the No. 5 tensile test piece. The r value is 15
% Tensile pre-strain and then measure by the three-point method.
Direction (rolling direction), D direction (45 ° direction to rolling direction)
And the average value in the C direction (90 ° direction to the rolling direction) is r
= Calculated as (r _L tens _{2r D + r C) / 4} . Regarding the evaluation of the secondary work brittleness resistance of the hot-rolled sheet, implementation and pass / fail judgment were performed in the same manner as described above. In addition, the evaluation of the secondary work brittleness resistance of the alloyed hot-dip galvanized steel sheet was performed and the pass / fail judgment was performed in the same manner as in the hot-rolled sheet. However, the test piece was punched out to a size of 50 mmφ, 2
After being squeezed out with a 4.4 mmφ ball-head punch, the obtained cup was cut at a depth of 21 mm and subjected to a test. Furthermore, the plating characteristics were determined by visually observing the occurrence of non-plating. “O” in Table 3 indicates that the plating characteristics were at a pass level at which there was no problem in practical use.
Indicates that the plating characteristics are not at the acceptable level.

[0052]

[Table 2]

[0053]

[Table 3]

From the evaluation results shown in Table 3, all of the hot-dip galvanized steel sheets (inventive examples) obtained according to the present invention have high tensile strengths of 400 MPa or more and comparative materials (comparative examples).
It can be understood that, in addition to the deep drawability, the secondary work embrittlement resistance and the plating properties are also excellent.

[0055]

As described above, according to the present invention, after the steel composition is adjusted, the hot-rolled sheet annealing is particularly performed while the black scale is adhered, so that the deep drawability, which is much better than the conventional one, and the resistance to 2 It is possible to obtain a high-strength hot-dip galvanized steel sheet having secondary working embrittlement and plating properties.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between r value and hot-rolled sheet annealing temperature in Nb-added steel and Nb-free steel.

FIG. 2 is a graph showing the relationship between Tcr and hot-rolled sheet annealing temperature in Mo-added steel and Mo-free steel.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Tetsuo Shimizu 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Inside the Mizushima Works of Kawasaki Steel Corporation (72) Inventor Takashi Sakata 1-term Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA02 EA04 EA15 EA16 EA17 EA18 EA19 EA23 EA25 EA27 EA28 EB02 EB03 EB09 FA01 FA02 FA03 FC02 FC03 FC04 FE01 FE02 FE03 FF02 FF03 FG03 GA05

Claims (4)

[Claims] C: 0.0005 to 0.008 mass%, Si: 0.1 to 1.5 mass%, Mn: 0.5 to 3.0 mass%, P: 0.02 to 0.2 mass%, S: 0.02 mass% or less, Al: 0.005 to 0.20 mass %, N: 0.01 mass% or less, B: 0.0005 to 0.008 mass%, Mo: 0.05 to 2.0 mass%, and Nb: 0.001 to 0.2 mass%, and 0.3 × (C / 12) ≦ Nb / 93 ≦ 3.0 × A steel slab containing (C / 12) within the range that satisfies (C / 12) with the balance being substantially the composition of Fe and unavoidable impurities. After heating and soaking at 950-1300 ° C, hot rolling at 650-1000 ° C After finishing,
Winding at 400-850 ° C, then annealing at a temperature range of 600-900 ° C for 10 minutes or more with the black scale attached,
After that, it is pickled, cold rolled at a reduction rate of 50 to 95%, and then recrystallized and annealed at 700 to 950 ° C. in a continuous hot dip galvanizing facility, followed by hot dip galvanizing.
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness resistance. 2. The steel slab according to claim 1, further comprising:
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized by having a composition containing Sb: 0.001 to 0.03 mass%. 3. The steel slab according to claim 1, wherein the steel slab further contains 0.002 to 0.05 mass% of Ti: Ti / 48 ≦ 1.5 ×
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized in that the composition contains a composition satisfying (N / 14 + S / 32). 4. The steel slab according to claim 1, wherein the steel slab further comprises 0.02 to 2.0 mass% of Cu and 0.02 to 2.
A method for producing a high-strength hot-dip galvanized steel sheet excellent in deep drawability and secondary work brittleness, characterized in that the composition contains one or two selected from 0 mass%.